This invention relates to a bio-based binding agent that is particularly effective when used in conjunction with a process disclosed herein for making impermeable agglomerates from finely divided minerals, such as coal, and for making insoluble composite materials from particulated lignocellulosics.
Many finely divided but otherwise useful materials are neglected or abandoned merely because they contain unwanted moisture; clearly, a low cost means for bonding such materials into durable products with a permanently reduced moisture content would enhance both their utility and value.
Providing such a means is a principal objective of this invention.
Earlier, attempts were made to adapt techniques disclosed in Applicant""s U.S. Pat. No. 5,371,194 xe2x80x9cBiomass Derived Thermoset Resinxe2x80x9d and U.S. Pat. No. 5,582,682 xe2x80x9cProcess For Making Cellulosic Compositesxe2x80x9d to the production of synthetic solid fuel from fines created during the mining of Wyodak coal in the western United States. Although acceptable quality syn-fuel products were made, the cost of energy required to remove all entrained water and to polymerize the binder was prohibitive. These results underscored the need for a less energy-intensive, and altogether cheaper method of making weather-resistant products from particulate materialsxe2x80x94a need now satisfied by the technology disclosed herein.
An exceptionally promising embodiment of this invention provides a long-sought alternative to coal mining""s most waste-intensive practicexe2x80x94the improvident discard of huge quantities of moisture-laden fine coal. In particular, this synthetic-fuel-making process has the capacity to open a new and profitable outlet for this energy-rich debrisxe2x80x94which at present creates environment tensions and financial burdens throughout the coal industry.
The novelty and economic merit of this new syn-fuel-making process, and the bio-based composition on which it relies, are unequivocally established by eliminating the need for the thermal energy invariably required by the prior art to dewater and dry coal fines, and to cure binders. The ability to dewater, shape and bond a variety of particulate feedstocks in a single continuous operationxe2x80x94without the input of thermal energyxe2x80x94is a distinguishing feature of processes employing this new bio-based composition. Coalescing, as used herein, means to quantify, shape, compress and express essentially all free water from a mixture of particulate, a wet-tack lubricant solution and a water-insoluble binding agent. When a chemically-inert particulate is agglomerated, coalescing may include both direct and indirect transfer of heat generated by compressive friction to the incipient agglomerate.
Essential to the dewatering method utilized in this invention is the presence on particulate surfaces of a substance, defined herein as a wet-tack lubricant, in very dilute solution. The preferred wet-tack lubricant is polyethylene oxide (PEO), a non-ionic water-soluble resin particularized by Union Carbide Corporation (UCI), Danbury, Conn. 06817, in brochure UC-876 5/95-5M. Relevant properties of PEO mentioned in the brochure include: xe2x80x9cLubricity, Friction Reduction, Water Thickening, Wet-Tack, and Shear-Thinning, and a high affinity for coal-fines, lignin and paper finesxe2x80x9d. PEO is known to flocculate lignocellulosics but not hydrophobic coal-fines, and it is hydrophilic but not a surfactant. While it is a relatively new chemical, the use of PEO in coal and paper processing has become extensive.
U.S. Pat. No. 4,322,219 of Burns discloses a process for removing moisture by contacting coalxe2x80x94either run-of-mine or pipeline slurry coalxe2x80x94with a dilute aqueous solution of PEO and allowing the moisture to evaporate; alcohol may be added to accelerate evaporation. The use of PEO solutions to xe2x80x9c. . . avoid the tendency of high-moisture low-rank coal to slack or degrade in size . . . xe2x80x9d is mentioned; however, no mention is made of small particles, or fines, and no suggestion that PEO could be used to facilitate the forcible expression of water from coal, or coal fines, or that de-watered coal, or coal-fines, could be agglomerated into a fuel productxe2x80x94with or without a binder.
The use of hydrophilic PEO in the process of present invention distinguishes it from U.S. Pat. No. 5,670,056 Yoon et al, which utilizes a hydrophobic reagentxe2x80x94preferably, mono unsaturated fatty esters or polysiloxane polymersxe2x80x94to aid mechanical means for dewatering coal-fines. No treatment beyond the dewatering of fine materials, such as agglomeration or bonding, is mentioned or suggested.
U.S. Pat. No. 5,658,357 of Liu et al. xe2x80x9cProcess For Forming Coal Compact Without A Binderxe2x80x9d uses the surface tension of water and the absence of air-bubbles to furnishxe2x80x9d. . . a binding effect which holds the carboniferous particles together and imparts mechanical strength in the compact . . . xe2x80x9d. If the surface-tension-producing water is removed from such a compact, disintegration quickly follows. Aside from reducing coal-log drag in a pipe line, the sole purpose of using a very dilute solution of PEO is reduction of the zeta potential to lessen electrostatic repulsion between particles in a coal slurry. In the present invention, the hydrophilicity, lubricity, thickening, shear-thinning, and fines affinity of PEO combine to facilitate particulate dewatering, densification and, unexpectedly, bonding during coalition.
Except for pellets made on a disc pelletizer, agglomerates made from mineral particulate by the process of the present invention have no need for the interim strength provided by PEO or supplemental heat; they are inherently insoluble and impermeable andxe2x80x94because they have been subjected to the frictional heat of compressive coalitionxe2x80x94strong and durable. But no appreciable heat is produced during disc pelletizing; therefore, although agglomerates made on these machines are impermeable and insoluble, supplemental heat is needed to obtain a peak strength product. As used herein, the terms insoluble and impermeable refer to the behavior of a product or substance with respect to water.
White""s U.S. Pat. No. 4,865,691 exploits a unique property of normally insoluble but water swellable polyvinyl alcohol (PVOH) particles: Swollen particles are dispersed in a dilute aqueous suspension of cellulosic fiber which, as excess water is drained, acts as a sieve retaining the swollen PVOH particles within the web. When heated, the entrapped swollen particles melt, dissolve into residual free water and diffuse into the web and, on cooling, solidify into a paper-reinforcing binder.
In U.S. Pat. Nos. 5,498,314, 5,328,567, and 5,800,675, Kinsley describes newer domestic grades of PVOH powder available from Air Products and Chemicals, Inc. (APCI) of Allentown, Pa. and specifies Airvol 125SF, 165SF, 350SF, 107SF, and 325SF as grades will allow the use of larger quantities of PVOH without undesirable side effects. None of the four aforementioned patents specify or suggest the use of PVOH in an unswollen state, or for a purpose other than paper-making.
Because PVOH is employed in a dissolved state as a coal-fines binder in the processes described in U.S. Pat. No. 4,787,913 of Goleczka, et al, and U.S. Pat. Nos. 4,586,936 and 4,863,485 of Schaffer, et al, they are easily distinguished from the present invention, wherein PVOH is used in an undissolved state. Neither swelling or dissolving PVOH powder is necessary to the present invention.
The xe2x80x98cook-outxe2x80x99 (dissolution) temperature of PVOH is specified by the manufacturer, APCI, as below 205xc2x0 F. (93xc2x0 C.) for all grades. In addition to the bio-based composition of the present invention and suitable grades of PVOH listed above, there are numerous water-insoluble binding agents, e.g., phenolic, acrylic, epoxy, thermosetting, or thermoplastic resins that melt or become temporarily soluble within the coalition temperature range (150-220xc2x0 F.) and would, therefore, be technically suitable for this duty. Without exception, however, such plastics have been found to be uneconomic.
Little if any frictional heat is created by the apparatus during coalition of lignocellulosic particulate to melt and disperse the PVOH powder. Product integrity and structural strength on an interim basisxe2x80x94without which the ostensibly-dry composite could not tolerate handling and shapingxe2x80x94must therefore be provided by PEO. For durability and peak strength, the ostensibly-dry composite material must be heated to melt and diffuse the PVOH, preferably while contained in a mold or press.
Coalesced composite material, with interim strength provided by PEO, may also be processed, e.g., molded, rolled, and shaped, and later subjected to heat sufficient to melt and disperse the binding agent, and dry the product. Like many other plasticizers known in the art, a small amount of PVOH will enhance composition tensile strength and flexibility. Likewise, small amounts of various lipids, such as paraffin, paraffin emulsions, and stearates and steric acids, will enhance composite hydrophobicity.
Any substantially water-insoluble PVOH powder hydrolyzed to a super, fully, or intermediate, extent is deemed suitable for use in this invention, with the higher viscosities (22-72 cps) preferred. The utility of undissolved PVOH powder is not mentioned in APCI""s brochure, nor is it disclosed in any other prior art. The phrase xe2x80x98substantially water-insolublexe2x80x99 means a substance that will not dissolve appreciably in water at room temperature, i.e., less than 25% w/w will dissolve in 30 minutes.
The use of soluble protein is old in the art of making paper coatings and adhesives; the process of Krinski, et al, disclosed in U.S. Pat. No. 5,766,331, for making a pigment binder exemplifies such use. In this process, a cation binding agent is added to inhibit formation of the insoluble gel created by addition of calcium oxide, or hydroxide, to a protein solution. This gelxe2x80x94termed herein a bio-based binding agentxe2x80x94is a basic and necessary element of the present invention, from which Krinski, et al, xe2x80x98331 is clearly distinguished by its teaching of the inhibition of gel formationxe2x80x94a contrary instruction.
In U.S. Pat. No. 5,543,164 of Krochta, et al, a method for making edible protein-based insoluble film and coating for foods is described. The present invention is readily differentiated from the process of Krochta et al xe2x80x98164, in which: A solution of denatured protein is applied to the exterior of a food item or made into a food wrapping filmxe2x80x94rather than incorporated as a necessary reagent in the composition of an industrial product; and, All means for denaturation, including heat, chemical or enzymatic treatment, may be employedxe2x80x94rather than the addition of calcium oxide or hydroxide, the only means found effective in the present invention. It is believed the alkaline ambiance created in products of the present invention by a relatively large amount of such a calcium compoundxe2x80x94in addition to effecting the irreversible denaturation of the protein solution and reducing SOx emissions during coal combustionxe2x80x94prevents the growth of micro-organisms that would otherwise eventually cause product deterioration.
Inasmuch as the binding agent composition disclosed herein stems from chemistry never previously used to create a particulate binding agent, it is easily distinguished from compositions of the prior art. Specifically, in a mixture of particulate with only a small amount of protein, alkali denaturation transforms the protein in situ into an insoluble gelatinous materialxe2x80x94which, during coalition, bonds the particulate and forms a moisture barrierxe2x80x94thereby yielding an insoluble and impermeable agglomerate. Impermeabilityxe2x80x94which is vital to the ability of syn-fuel to retain a high BTU level and survive all-weather storage and transportxe2x80x94is verified with a simple water-soak test: No weight gain is observed after an agglomerate made by the process of this invention has been immersed in water for 24 hours.
The process of this invention is based on the discovery that a dewatered product with insoluble inter-particle bonds can be obtained when an alkali, such as lime, is admixed with a mixture of moist PEO-treated particulate and a soluble protein, and the resulting admixture is coalesced. As a result of protein denaturation, a gelatinous insoluble substancexe2x80x94which acts as a binding agent and a permanent sealantxe2x80x94is formed in situ in the coalesced product. Following PEO-facilitated dewatering at the outset of particulate coalition, this binding agent provides integrity and strength. If the coalesced particulate is an inert mineral, such as coal fines, the frictional heat that accompanies compressive coalition evaporates residual moisture from the agglomerate making it stronger and more durablexe2x80x94in addition to being insoluble and impermeable.
Use of this procedure with lignocellulosic particulate yields an analogous product; however, because only a negligible amount of frictional heat is generated during cellulosic coalition, supplemental heat is required to obtain a substantially dry composite. Moreover, because the binding agent yields an insoluble but not impermeable composite, moisture can be re-absorbed into the lumens and pores of lognocellulosicxe2x80x94unless such penetration is precluded by the addition of a hydrophobizing ingredient to the feedstock mixture, or application of a water-repellant coating to the composite product.